A motion guide apparatus is used to guide the linear motion (rectilinear or curvilinear motion) of a movable body such as a table. The motion guide apparatus includes a guide rail attached to a base, a carriage assembled to the guide rail so as to perform linear motion, and multiple rolling elements disposed between the guide rail and the carriage so as to perform rolling motion. When the carriage moves relatively to the guide rail, the multiple rolling elements perform rolling motion therebetween. The use of the rolling motion of the rolling elements allows a highly accurate, smooth guidance of the movable body.
Fastening member insertion holes are provided at constant pitch in a length direction to the guide rail. A fastening member such as a bolt for fastening the guide rail to the base is threaded through the fastening member insertion hole. When foreign substances such as dust are accumulated in the fastening member insertion hole, even a seal provided to the carriage cannot scrape out the foreign substances. In order to prevent foreign substances from entering the carriage and hindering smooth rolling motion of the rolling elements, the guide rail is attached by the fastening members to the base, and then the fastening member insertion holes of the guide rail are blocked by a cap (refer to, for example, Patent Literature 1).
A cap provided with a plurality of protrusions around a disc-shaped cap body is known as a known cap in Patent Literature 1. The cap is temporarily placed on the fastening member insertion hole, and then driven into the fastening member insertion hole using a hammer or the like. The protrusion of the cap is provided with interference. The cap is fixed by the interference to the fastening member insertion hole.